The Effect Research Of Droplet Condensation And Evaporation On The Surface Of Cold Radiant Plate

Compared with the traditional air conditioning based on convective heat transfer,radiant cooling system has the advantages of less space occupancy,lower energy consumption and higher comfort.It is a new direction of air conditioning technology development.When the environmental conditions change,the cold radiation heat transfer surface is easy to condense,which has become the main reason to limit its engineering application.The study of condensation on the surface of cold radiant panel,especially its droplet condensation and evaporation characteristics under different influencing factors,has important guiding significance for the suppression of condensation on cold radiant panel.In this paper,the heat transfer process between the base of the cold radiant plate and the droplet,the droplet surface and the saturated air boundary layer,the saturated air boundary layer and the mainstream air layer is analyzed.The mathematical model of heat and mass transfer in droplet condensation and evaporation process with constant apparent contact angle is established.The experimental station of droplet condensation evaporation was built,and the smooth aluminum alloy surface and laser-etched aluminum alloy surface were made as the surface materials of the new cold radiation plate.The whole process of the two material surfaces from independent growth in the initial stage of droplet condensation,condensation and growth in the middle stage to abscission or evaporation was observed.The mathematical model of droplet condensation heat and mass transfer is verified by visual experimental results.Using the mathematical model of heat and mass transfer in the process of droplet condensation and evaporation,taking a single droplet as the object,the factors affecting the droplet growth rate are analyzed.The results show that the partial pressure of water vapor in the air and the base temperature of the cold radiation plate have the greatest influence on the droplet growth process,followed by the size of the droplet contact angle on the surface of the cold radiation plate.When the thermal conductivity of the cold radiant plate material is lower than 393W·m^-1?K^-1,the plate thickness is less than 2mm and the microstructure height is less than 30μm has little effect on the condensation rate.The independent droplet growth model is further extended to the coalescence growth model.The time of droplet bouncing off or gravity falling off under different apparent contact angles,working temperature of cold radiant plate and partial pressure of ambient water vapor are explored,as well as the changes of droplet density,droplet size and droplet coverage in this process.The results show that the coalescence growth rate of droplets is three times higher than the independent growth rate of droplets.The falling off time of superhydrophobic surface in the state of coagulation and bouncing is much earlier than the state of gravity.The droplet bouncing off radius depends on the contact angle,and the bounce occurrence time is most affected by the surface temperature of the cold radiation plate,while the main influencing factors of droplet density,droplet size and droplet coverage are the surface temperature of the cold radiation plate and the ambient water vapor partial pressure.In the state of the cold radiant plate apparent contact angle with 150°,the working temperature with 17°C,and the ambient water vapor partial pressure with 2300pa,the falling off time of condensate droplets is about 10min.In the practical application of cold radiation air conditioning,this time can be used as a reference.In the early stage of air conditioning operation,the methods of attaching jet,air dehumidification and increasing the surface temperature of cold radiation plate can be used to inhibit the condensation on the surface of cold radiation plate.